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C:\Documents and Settings\Alan Smithee\My Documents\MOTM L`x1//6Lhmdq`knesgdLnmsg9Aqnnjhsd This month marks several firsts: our first mineral from an Alpine-cleft deposit, our first from Pakistan, and the first time we’ve seen so many excellent crystals of this rare mineral! OGXRHB@K OQNODQSHDR Chemistry: TiO2 Titanium Dioxide, often containing iron Class: Oxides Subclass: Simple Oxides Group: Brookite Crystal System: Orthorhombic Crystal Habits: Tabular to platy, thin in one direction; sometimes sheet-like, with pseudo-hexagonal cross- sections, parallel striations, and small, brilliant termination faces. Less commonly as black, opaque, six-sided bipyramids (variety arkansite). Twinning rare. Crystals usually less than one-quarter inch in size. Color: Brown, yellow-brown, amber, orange, and reddish-brown. Luster: Submetallic to adamantine Transparency: Transparent to translucent and opaque Streak: Yellowish-white Cleavage: Poor in one direction Fracture: Subconchoidal, brittle Hardness: 5.5-6.0 Specific Gravity: 4.10-4.14 Luminescence: None Refractive Index: 2.583-2.705 Figure 1. Brookite crystal Distinctive Features and Tests: Submetallic luster, relatively high density, reddish-brown color, frequent association with rutile, anatase [both polymorphs of titanium dioxide, TiO2], and quartz [silicon dioxide, SiO2] in alpine-cleft-type deposits. Dana Classification Number: 4.4.5.1 M @L D The name of this month’s mineral is pronounced just as it is spelled—BROOK-ite, and is named after the English crystallographer and mineralogist Henry James Brook (1771-1857). Brookite is also known as “arkansite,” “eumanite,” “jurinite,” “pyromelane,” “acid titanium,” “ortho-rutile,” and “ortho-titanium.” European mineralogists refer to brookite as “brookita” and “brookit.” BNL ONRHSHNM Titanium and oxygen combine to form three distinct minerals, and possible a fourth, as we will see. We featured the most common of these, rutile, in June 1999, as small specimens with golden radiating needles from Bahia, Brazil. We never thought we would be able to feature the other two, brookite and anatase, but now that we have done one, perhaps the other will become available in the future as well! Brookite’s chemical formula, TiO2, identifies its elemental components as titanium (Ti) and oxygen (O). Titanium makes up 59.94 percent of brookite’s molecular weight, with oxygen accounting for the remaining 40.06 percent. The molecular weight of a mineral is the sum of the atomic weights of its constituent elements. The atomic weight of titanium is 47.88 and that of oxygen is 16.00. Brookite’s molecular weight is therefore calculated as 47.88+(2x16.00)=79.88. To determine the weight percentage of titanium in brookite, divide the atomic weight of the single titanium ion by brookite’s molecular weight Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 1 L`x1//6Lhmdq`knesgdLnmsg9Aqnnjhsd (47.88/79.88=.5994). To determine the weight percentage of any elements in any mineral compound, simply obtain the atomic weights of the mineral’s elemental constituents from a reference source and complete this calculation. As a simple oxide, brookite consists of only a single metal combined with oxygen. The brookite molecule achieves electrical stability through the balance of the +4 charge of its titanium (Ti4+) cation (positively charged ion) with the collective -4 charge of its double-oxygen (2O2-) anion. Brookite crystallizes in the orthorhombic system, which has three mutually perpendicular axes of different lengths. When one axis is notably shorter than the others, as in brookite, the crystal is tabular, platy, or sheet-like. The brookite lattice consists of a three-dimensional arrangement of staggered pinacoidal structures with titanium ions at the corners and oxygen ions occupying the face or interior positions. The lattice is joined together mainly by strong, covalent, titanium-oxygen bonds that give brookite a substantial hardness of 5.5-6.0 and no pronounced cleavage planes. Because its oxygen ions do not completely shield its titanium ions, brookite exhibits a diagnostic submetallic luster. Brookite’s considerable density (specific gravity 4.10-4.14) is due to the fact that titanium, a moderately heavy metal (atomic weight 47.88) comprises 59.94 percent of its molecular weight. Unlike most minerals, brookite occurs only as crystals and never in compact masses or aggregates. It is usually associated with its polymorphs rutile and anatase (see “The Polymorphs of Titanium Dioxide”) and attached to cavity walls in weathered igneous rocks. Most brookite occurs in alpine-cleft-type deposits (mineralized fissures in igneous rock) in association with anatase, rutile, titanite [calcium titanium oxysilicate (sphene), CaTiOSiO4], quartz [silicon dioxide, SiO2], albite [sodium aluminum silicate, NaAlSi3O8], and calcite [calcium carbonate, CaCO3]. In metamorphic rocks, brookite is attached to the walls of clefts in gneiss and crystalline schists. Lesser amounts of brookite form by hydrothermal alteration. Brookite is also found as minute, isolated crystals in sedimentary rocks. Brookite’s considerable density enables it to concentrate gravitationally in alluvial placer deposits in association with gold and other heavy minerals. Brookite is an idiochromatic (self-colored) mineral, meaning that its characteristic reddish-brown color is due to its inherent chemistry and crystal structure. When pure, brookite has a high degree of transparency and a bright reddish-brown color. Impurities, usually of iron, create color variations and reduce transparency. Brookite’s abilities to refract and disperse light are among the highest in the Mineral Kingdom. Refraction, the ability to bend light, is measured by index of refraction (R.I.), which is the ratio between the speed of light in air and in a crystal. The R.I. of most mineral crystals and gemstones falls between 1.4 and 1.9. Higher numerical values indicate greater degrees of refraction and correspond directly to brightness and brilliance in gems. Diamond (carbon, C), known for its brilliance, has an unusually high R.I. of 2.417- 2.419—which is still considerably less than that of brookite (R.I. 2.583-2.705). Dispersion refers to the ability of a crystal or gemstone to disperse white light into its spectral colors. Dispersion is expressed numerically as the difference between the red and violet refractive indices. The particularly high color dispersion of diamond (dispersion index 0.044) produces a beautiful display of color called “fire.” Yet the dispersion index of brookite—0.269—is six times higher. The ability to refract and disperse light is dependent primarily on crystal structure and to a lesser extent on density and chemistry. Unfortunately, even with these properties, brookite makes poor gemstones, as will see. The Dana subclassification number 4.4.5.1 first identifies brookite as a simple oxide (4). The 4+ 4+ subclassification (4) next defines it by the general formula A O2, in which “A ” is a quadvalent metal cation. Finally, brookite is a member of the brookite group (5) as the first (1) and only member. Bnoxqhfgs 1//6 ax Qhbg`qc % Bgdqxk Rhsshmfdq L hmdq`k ne sgd L nmsg Bkt a 0 66/ N qu hkkd @ u dmt d B`l aqh`+ B@ 8 2 3 17 0 ,7 //,8 3 0 ,4 4 8 3 v v v -l hmdq`knesgdl nmsgbkt a-nqf 2 L`x1//6Lhmdq`knesgdLnmsg9Aqnnjhsd THE POLYMORPHS OF TITANIUM DIOXIDE The three polymorphs of titanium dioxide (TiO2) are brookite, rutile, and anatase. These polymorphs have an identical chemistry, but different crystal structures. Rutile and anatase both crystallize in the tetragonal system, while brookite crystallizes in the orthorhombic system. Although rutile and anatase are both tetragonal minerals, their crystal habits are quite different. Rutile forms prismatic to acicular crystals with dipyramidal terminations and is often twinned, while anatase forms only tabular or dipyramidal crystals and does not twin. The following table points out the similarity in chemistry and the physical differences between the titanium-dioxide polymorphs. Chemistry Crystal System Density Hardness Twinning Cleavage Rutile TiO2 tetragonal (prismatic) 4.25 6.0-6.5 common 2/good Anatase TiO2 tetragonal (tabular) 3.9 5.5-6.0 none 3/perfect Brookite TiO2 orthorhombic 4.1 5.5-6.0 none 1/poor The titanium-dioxide polymorphs crystallize at different temperatures: Anatase crystallizes first and at very high temperatures, brookite next and at moderate temperatures, and rutile last and at low temperatures. Anatase and brookite crystallize only in precise conditions of temperature, pressure, and chemistry. Rutile crystallizes over a broader range of conditions, which explains why it is by far the most abundant of the three polymorphs and comprises most of the titanium dioxide in the Earth’s crust. Rutile is the only stable polymorph of titanium dioxide; when subjected to high temperatures, both anatase and brookite undergo structural rearrangement of their lattices and convert to rutile. Mineralogists are currently studying a very rare, not-yet-named mineral that may be recognized as the fourth polymorph of titanium dioxide. Referred to as “TiO2B,” this form of titanium dioxide has been found only in such shock-induced, very-high-pressure environments as meteor-impact sites. Its very tight lattice structure and high density reflect its formation under conditions of extreme pressure. BNKKDBSHM F KNB@KHSHDR Our brookite specimens come from what is currently the world’s finest source for this mineral, an alpine- cleft deposit in the Khârân District of the Balochistan Province of western Pakistan. Brookite is also collected in Russia at the Dodo Mine in the Subpolar Urals at Tyumenskaya Oblast’ in the Western- Siberian Region, and at the Lovozero and Khibiny massifs on the Kola Peninsula in the Murmansk Oblast’ of the Northern Region. In England, exceptional specimens occur at the Cwmorthin and Manod quarries at Gwynedd in Wales, and the Shap Pink Quarry at Eastern Fells in Cumbria.
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